Piecing together Fukushima

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TO CALL it a hot ticket might, in the circumstances, seem a tad tasteless. But no session at this year’s International Conference on Advances in Nuclear Power Plants, held in the south of France, was as well attended as the late-running special plenary hastily arranged to provide an update on the nuclear disaster at the Fukushima Dai-ichi plant in Japan and its attendant radioactive leaks. Akira Omoto of the University of Tokyo, an industry veteran who is a member of Japan’s Atomic Energy Commission, Hideki Masui, who works on seismic hazards at TEPCO, the plant’s operator (where Mr Omoto has also been employed), and Kenichi Sato of GE Hitachi, which makes the type of reactor involved, provided a range of updates and insights (each of them was keen to stress that he was speaking in a personal capacity).

Much of what they had to say was already familiar to many in the audience; the subject has been closely followed in the nuclear industry. But the sheer power of the story, some fresh details, and the continuing mysteries about some of its particular twists, kept the audience captivated. And hearing the story from people as in the know as anyone is likely to clarify some things—including which questions need further clarification.

Mr Omoto stressed that it was not the earthquake, nor the tsunami, which doomed the plant, but the combination of the two. The earthquake in and of itself did not do too much damage; it shook the reactor buildings slightly more than they were designed to be shaken, but they were built well and seem not to have suffered much harm. The three reactors running at the time shut down as they were meant to. But the earthquake did one crucial other thing: it knocked out the connections which brought electricity from the grid to the power plant. After the earthquake, the plant was on its own. Its sister plant, Fukushima Dai-ni, stayed on the grid through the earthquake and the tsunami; if it hadn’t then things there, too, might taken a drastic turn for the worse.

The earthquake’s effects had been only a bit worse than Fukushima’s designers had expected; the tsunami which arrived just under an hour later was much worse. Fukushima Dai-ichi, which sits ten metres above sea level, was originally designed to withstand a 3.1 metre wave. A “Severe Accident Management” review in 2002 increased estimates of the tsunami threat at all Japan’s power plants, raising the average wave height they needed to be designed to cope with to about double the height of the biggest waves in the historical record. The maximum wave to be planned for at Fukushima was pushed up to 5.7 metres. The March 11th wave was about 15 metres. It smashed the plant’s sea water intake systems, flooded electrical switching facilities and diesel generators, and carried off diesel fuel tanks; because of the layout of the site the water did considerable further damage as it flowed back out.

All this meant that the plant no longer had the facilities to cool its reactors which, though shut down, were still generating plenty of heat that had to be dealt with. Cooling systems that didn’t require alternating current—which is what the grid or the diesel generators would have provided—worked for a while, but eventually failed. The reactors began to overheat and damage themselves. One implication of this is that designers should think about external challenges to their reactors coming in pairs—and not necessarily pairs which share a common cause, like earth tremors and tsunamis.Japan's nuclear disaster: Piecing together Fukushima | The Economist